Gamma irradiation induced surface modification of (PVC/HDPE)/ZnO nanocomposite for enhancing the oil removal and conductivity using COMSOL multiphysics

Blend nanocomposite film was prepared by loadings of irradiated ZnO in ratios of (5 wt%) inside the PVC/HDPE matrix using a hot-melt extruder technique. The physical and chemical properties of the irradiated and unirradiated ZnO samples are compared. The Vis–UV spectrum of ZnO shows an absorption peak at a wavelength of 373 nm that was slightly red-shifted to 375 nm for an irradiated sample of ZnO at a dose of 25 kGy due to the defect of crystal structure by the oxygen vacancy during gamma irradiations. This growth of the defect site leads to a decrease in energy gaps from 3.8 to 2.08 eV. AC conductivity of ZnO sample increased after the gamma irradiation process (25 kGy). The (PVC/HDPE)/ZnO nanocomposites were re-irradiated with γ rays at 25 kGy in the presence of four different media (silicon oil, sodium silicate, paraffin wax and water). FTIR and XRD were performed to monitor the changes in chemical composition. The new peak at 1723 cm−1 attributed to C=O groups was observed in irradiated (PVC/HDPE)ZnO samples at only sodium silicate and water media. This process induced new function groups on the surface of the (PVC/HDPE)/ZnO blend sample. This work aims to develop (PVC/HDPE)ZnO for oil/water separation. The highest oil adsorption capability was observed in samples functionalized by C=O groups based on the different tested oils. The results suggest that the surface characterization of the (PVC/HDPE)/ZnO can be modified to enhance the oil adsorption potential. Further, the gamma irradiation dose significantly enhanced the AC conductivity compared to the unirradiated sample. According to COMSOL Multiphysics, the irradiated sample (PVC/HDPE)ZnO in water shows perfect uniform electric field distribution in medium voltage cables (22.000 V).


Preparation of (PVC/HDPE)/ZnO by melt extruder methods. Polyvinyl chloride was melt-mixed
with HDPE in the ratio (30/70) wt/wt% using a twin screw extruder (CTW100P; Haake Poly lab Rheomix, Germany). The content of ZnO was 5wt% and adding in the meld blends of PVC and HDPE and the rotating screw speed in the extruder was 120 rpm. The extrudates obtained from the twin screw extruder were two-roll milled (Lab Tech Engineering Co., Bangkok, Thailand) at 170 °C for 7 min before compression-molding with a hot press (Lab Tech Engineering Co., Bangkok, Thailand) at 170 °C with a pressure of 150 kg/cm 2 for 4 min. The moulded (PVC/HDPE)/ZnO composite was cut into test pieces for further experimental evaluation. www.nature.com/scientificreports/ Preparation of ZnO nanoparticles. A typical experiment created ZnO nanoparticles using the traditional sol-gel process. The solution, A of zing salt, was created by dissolving 20.196 g (0.10 mol) of zinc acetate in 600 mL of water/ethanol in a ratio 80/20 v/v% and stirring it for 60 min at room temperature. A solution of 0.20 mol of oxalic acid dehydrates was obtained by dissolving 2.520 g in 800 mL of water/ethanol in ratio 80/20 v/v% and stirred at a temperature of 50 °C for 60 min to create Solution B. Warm solution A was constantly mixed for one hour while solution B was added dropwise. A white sol was obtained, aged to create a gel, and then dried for 24 h at 100 °C. ZnO was finally produced using thermal processing at calcination temperatures of 600 °C for 3 h.
Gamma irradiation-induced Surface modification of (PVC/HDPE)/ZnO samples. Another advantage of radiation-induced surface modification in four different media (silicon oil, sodium silicate paraffin wax and water) is that it enables tailored modifications ranging from surface to bulk of backbone polymers, unlike photo-and plasma initiation, which imparts surface modification only. The modification process can improve the hydrophilicity or hydrophobicity of (PVC/HDPE)/ZnO samples or their conductivity or modify its oil adsorption. Sheets of irradiated (PVC/HDPE)/ZnO samples are cut into strip samples and re-irradiated with γ rays at 25 kGy in four different media (silicon oil, sodium silicate paraffin wax and water). The irradiation process is carried out under ambient conditions and a dose rate of 0.67 kGy/h is maintained by using a Co-60 source (Irradiation is performed NCRR, AEAE).
Characterization.  After an irradiation dose of 25 kGy, the characteristic absorption peak of the ZnO NPs at wavelength of 373 nm was slightly red-shifted relative to the absorption maximum of 375 nm. It may be due to the defect of crystal structure by the oxygen vacancy during gamma irradiations. This growth of the defect site leads to a decrease in energy gaps to become 2.08 from 3.8 eV with an irradiation dose of 25 kGy. The development of the defect state could be caused by the reduction of compressive stress in the ZnO films. The reduced compressive stress of irradiated ZnO may be attributed to the well-aligned ZnO hexagonal nanoparticles 37 . Figure 2 shows the XRD analysis of un-irradiated and irradiated ZnO nanoparticles at 25 kGy. The XRD curve shows 7 intensity peaks located at 2θ = 31.57°, 34.13°, 36.00°, 47.57°, 56.45°, 62.72° and 67.70°, confirming the hexagonal wurtzite structure of ZnO NPs according to 38  www.nature.com/scientificreports/ The effect of gamma irradiation at dose of 25 kGy on the contact angles of ZnO was investigated. Figure 3 shows the wettability character of un-irradiated and irradiated ZnO nanoparticles by measuring the contact angles of the water drop. The contact angle of the irradiated sample is increased from 54.36° to 65.25° compared to the blank sample. The increased contact angle of the irradiated sample is due to the increased ZnO orientation and the well-aligned ZnO hexagonal nanoparticles, as confirmed by band gab data analysis and XRD data analysis. Figure 4 shows the variation in AC conductivity as a function of ln frequency at two irradiation doses (0 to 25) kGy for ZnO nanoparticles. It is observed that the two samples exhibit different AC conductivity phases with different values over the whole range of measured frequency. The Ac conductivity of the irradiated sample is higher than unirradiated samples. The possible increase in defects on ZnO's crystal structure and the expansion  www.nature.com/scientificreports/ of conduction routes between ZnO particles are linked to the growth in AC conductivity. The bulk conductivity may rise due to more charge carriers being able to "jump" by tunneling, and it also rises with gamma irradiation exposed 41,42 . Table 2 43 found that the surface roughness of poly(lactic acid) (PLA) fibers can be controlled by increasing the content of Zn 2+ in metal framework of (ZIF-8). Oil adsorption is accelerating due to the higher roughness of (PLA/ZIF-8) fiber surface. Haq et al. 44 prepared oil adsorbent resin system includes (unsaturated polyester + epoxidized soybean oil + nanoclay) with varying nanoclay amounts. He and coauthor obviously that the surface's roughness is increased due to the addition of nanoclay. Barroso-Solares et al. 45    The surface hydrophobicity modification of (PVC/HDPE)/ZnO samples. The obtained compatible blends of (PVC/HDPE) ZnO irradiated at different media (paraffin wax, silicon oil, sodium silicate and water) as an efficient method for the hydrophobicity modification of the surface of blends to use in oil/water separation. Surface modification is modifying the surface of (PVC/HDPE) ZnO blends by bringing chemical and physical properties different from the blank sample. The physicochemical properties of surface-modified (PVC/ HDPE) ZnO blends were performed using other characteristics such as FTIR, mechanical properties and XRD. The chemical structure of the surface modification of (PVC/HDPE)ZnO blend films using FTIR spectroscopy is shown in Fig. 6. The two FTIR peaks located at 2918 cm −1 and 623 cm −1 are characteristic peaks of C-H and C-Cl bonds of PVC molecules, respectively. The peak at 1290 cm −1 is attributed to trans CH 3 in polyethylene molecules 46 . The peak at 1723 cm −1 attributed to C=O groups was observed in unirradiated (PVC/HDPE)ZnO samples and irradiated samples at sodium silicate and water media. On the other hand, the C=O groups completely disappeared in case (PVC/HDPE) ZnO samples were irradiated in paraffin wax and silicon oil. The peak intensity of 1290 cm −1 was increased in the case of the irradiated sample at silicon oil and sodium silicate media due to the formation of Si-CH 3 at 1250 cm −147-49 . Figure 7 shows the X-ray diffraction (XRD) patterns of the blank and the modified surface of irradiated (PVC/HDPE) at the dose of 25 kGy. It is noted that, the XRD peaks of ZnO is not clear in Fig. 7a,b samples of (PVC/HDPE) at the dose of 0 kGy and 25 kGy, respectively due to the surface modification of (PVC/HDPE)/ ZnO leading to concentration the nanoparticles in the surface of plastic sheet similar to the bulk of the samples in Fig. 7c-f. The characteristic XRD peaks of HDPE are located at ~ 21° and 23°, which correspond to the typical semi-crystalline nature of orthorhombic unit cells of (110) and (200) reflection planes, respectively 50-52 . Figure 7 confirms plane (110)'s diffraction peaks are almost changing in all modified (PVC/HDPE) samples. This means the modified surface is placed on the plane (110) of HDPE 53 . Also, Fig. 7 shows a very broad XRD peak of PVC ranging from 14° to 24°; this indicates that PVC is amorphous 54 . Also, XRD in Fig. 7 38 .

Results and discussion
From the bar graph Fig. 8, it can be perceived that the force (N), elongation (mm) and Young's modules (MPa) of unirradiated (PVC/HDPE)ZnO blends are much more than all irradiated (PVC/HDPE)ZnO blends samples. The Young's modulus and force of blank (PVC/HDPE)ZnO (0 kGy) and at (25 kGy) were 51.7 MPa, 193.66 N and 24.22 MPa, 104.3 N, respectively. After gamma irradiation, Young's modulus and force were observed to be decreased due to the defect state of the atomic bonds in blends induced by gamma irradiation. On the other hand, Young's modulus and force were observed to be decreased in all treated samples after surface treatment. These results justify that the surface modification contributed to changes in the surface's chemical structure; falls in mechanical properties can be due to the low interfacial bonding between the bulk and surface of the blend chain. The intense interfacial bonding between the bulk and surface of the treated blend chain may cause an elongation increase compared to blank samples. Further, the mechanical properties of the (PVC/HDPE)ZnO blend gamma irradiation treated in Na-Si exhibit higher Young's modulus and force value than other treated (PVC/HDPE)ZnO blend samples. This may be attributed to a harder physical bonding effect and the formation of weak bonds on the (PVC/HDPE)ZnO blends surface.

Effects of gamma irradiation and surface modifications on the removal of oil. Oils absorb on
surfaces due to a surface phenomenon. As a result, adsorbents have vast surface areas, most of which are interior surfaces that enclose the massive pores and capillaries of highly porous materials. As shown in Fig. 9 the oil-adsorbed power of (PVC/HDPE)/ZnO is decreased after irradiation. One of the most reasons is the porous adsorbents become smaller and reduce their capability for efficiently adsorbing due to gamma irradiationinduced cross-linked reactions. While after surface modification, the four modified (PVC/HDPE)/ZnO show a wide spectrum of adsorption capability for the six different kinds of oils. The performance characteristics of adsorbents largely relate to their intraparticle properties such as surface chemical compositions and surface hydrophobicity. The hydrophobic surface area of modified samples and the functionalised distribution of groups concerning pore size and roughness surface generally are primary determinants of adsorption capacity. On the other hand, the density of removal oil and API number (American Petroleum Institute number is used to identify the oil and gas wells) should be considered. The effect of API on the adsorption potential of oil was in inverse relations ship. As show in Fig. 9 the percentage adsorption capacity of oils was observed to increase in direct proportion with a decrease in API value and at an inverse proportionality with low functionalized sites in modified (PVC/HDPE)ZnO samples due to the presence of functional groups found in crude oils 55 . According to previous literature, crude oils have other groups containing -C=C-, C=O and OH with excess sulfur and nitrogen 56 . For castor oil, the adsorbent sample of (PVC/HDPE)/ZnO irradiated at a 25 kGy in water media increased from 73 to 157% compared to the blank sample irradiated at 25 kGy. As depicted in Fig. 9, the adsorption uptake was observed to be changed depending on the kind of adsorbent. As the functionalized sites on the surface were increased from irradiated samples at 25 kGy in water and NaSi solutions, the adsorption uptake www.nature.com/scientificreports/ of oil increased consistently. The adsorption uptake of motor oil increased from 26%, 56%, 64%, 73% and 75% for modified irradiated samples in air, wax, Si oil, water and NaSi. The highest adsorption capability of oils was observed in samples functionalized by C=O groups. The availability of the functional group sites on the adsorbed surface enhanced the oil removal capability. The results suggest that the surface characterization of the (PVC/ HDPE)/ZnO can be modified to enhance the oil adsorption potential.   Figure 10 shows the AC conductivity of (PVC/HDPE)/ZnO, which varied in the same behavior independently on the surface modification and the type of function group formation. However, it should be noted that each sample's percolation threshold depends on the gamma irradiation condition in different solutions. Figure 10 shows the lowest conductivity of un-irradiated samples of (PVC/HDPE)/ZnO seemed to increase twice after irradiation at a 25 kGy. This is due to the gamma irradiation-induced defect by atom displacement that increased the electron transition after the formation of holes. Additionally, dehydrochlorination of PVC molecules brought on by gamma irradiation may result in the formation of conjugated double bonds. Throughout these conjugations, the electrons then become mobile. Consequently, electron mobility contributes to the material's overall conductivity 57,58 .
On the other hand, the two irradiated samples of (PVC/HDPE)/ZnO in water and NaSi solutions had higher conductivity values for the same given frequency. This is due to forming new function groups in the modified surface detected by FTIR data. The functional groups can serve as a host matrix for electron transition.
Simulation and modeling of electric field distribution inside irradiated (PVC/HDPE)-ZnO) nanocomposite at the dose of 25 kGy. To simulate the distribution of the electric field in MV cables, utilise COMSOL Multiphysics. The distribution of the electric fields inside the (PVC/HDPE)ZnO unirradiated sample is depicted in Fig. 11. The electric field distribution inside the sample is not uniform at 1 mm of arc length. The electric field distribution for irradiation (PVC/HDPE)ZnO/water is becoming uniform and steadily www.nature.com/scientificreports/ declines from the interior to the outside, as shown in Fig. 12. This is due to the new C=O function groups ability to maintain an even electrical field while lowering electrostatic tension.

Conclusions
• ZnO was irradiated at dose of 25 kGy for improved it physiochemical properties, The Vis-UV spectrum of ZnO shows an absorption peak at a wavelength of 373 nm that was slightly red-shifted to 375 nm for an irradiated sample of ZnO at a dose of 25 kGy due to the defect of crystal structure by the oxygen vacancy during gamma irradiations. • This growth of the defect site leads to a decrease in energy gaps from 3.8 to become 2.08 eV.
• The contact angle of the irradiated sample is increased from 54.36° to 65.25° compared to the blank sample.
• Surface modification of the (PVC/HDPE)ZnO with gamma irradiation treatment was investigated.
• The gamma irradiation process at dose of 25 kGy was performance in four different media such as (paraffin wax, Si oil, NaSi and water) to improve the surface by a functionalized group of PVC/HDPE)ZnO samples. • FTIR spectra revealed chemical change on the (PVC/HDPE)ZnO surface after gamma irradiation treatment in water and NaSi. • The C=O peaks appeared in the expected FTIR, confirming the sample surface's alteration.
• The surface of the (PVC/HDPE)ZnO sample enhances its oil removal capability compared to blank samples.
• The adsorption uptake of motor oil increased from 26%, 56%, 64%, 73% and 75% for modified irradiated samples in air, wax, Si oil, water and NaSi. • The highest adsorption capability of oils was observed in samples functionalized by C=O groups.

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.